Abnormal bleeding is a common clinical problem in adolescents, with the vast majority of cases resulting from anovulatory menstrual cycles. Although most episodes of abnormal vaginal bleeding do not cause acute medical complications, bleeding can be traumatic for young patients and their families. Health care providers with a solid knowledge of menstrual physiology and a thorough approach to differential diagnosis can evaluate and appropriately manage adolescents. In this article, we review the maturation of the hypothalamic ovarian axis, the most frequent etiologies of abnormal bleeding, as well as other less common causes and discuss diagnosis and treatment modalities.